1. Field of the Invention
The present invention relates to guided-wave optical branching components and guided-wave optical switches which are preferably used in the optical communication field. More specifically, the present invention relates to guided-wave optical branching components wherein the wavelength dependence of the power coupling ratio is reduced, and further relates to guided-wave optical switches which can switch optical signals in a wide wavelength region with reduced wavelength dependence.
2. Description of the Prior Art
For development of optical fiber communications, the development of various optical circuit components such as optical branching/combining components, optical multiplexers/demultiplexers, optical switches, and the like, is essential in addition to the fabrication of optical fibers, photodetectors, and light emitting devices of high quality and low cost. Above all, optical branching components are the most basic optical component: optical branching components having various branching ratios (coupling ratios) such as 50 percent, 20 percent, or a few percent are required. In particular, optical branching components of little wavelength dependence in a wide wavelength region are earnestly required.
Optical branching components are also called optical couplers, and are classified as the following three types: (1) bulk-type branching components; (2) fiber-type branching components; and (3) guided-wave type branching components.
The bulk-type branching components are constructed by arranging microlenses, prisms, interference-film filters, etc., and have little wavelength dependence. Although the bulk-type branching components can be put into practical use to some extent, the components require a long time for assembly and adjustment, and present some problems with regard to long-term reliability, cost and size.
The fiber-type branching components are fabricated, using optical fibers as constituent material, through processes which may include grinding and polishing, fusing and elongating. Although this type of component makes it possible to produce branching components of reduced wavelength dependence, the fabrication process requires skill, and is not suitable for mass production because of lack of reproducibility.
In contrast, guided-wave type branching components have the advantage that they can be constructed on flat substrates in large quantities through the photolithography process. Hence, they attract attention as a promising type of branching component which can be reproduced and integrated as compact parts.
FIG. 1 is a planar view exemplifying a configuration of a conventional (2.times.2) guided-wave type branching component. In FIG. 1, two optical waveguides 2 and 3 are formed on a flat substrate 1. A part of the optical waveguide 2 and a part of the optical waveguide 3 are brought into proximity with each other to form a directional coupler 4. The directional coupler 4 is designed in such a way that an optical signal launched into a port 5 is branched to ports 6 and 8 to be outputted. Although the power coupling ratio of the directional coupler 4 can be specified to a desired value at a particular desired wavelength, the wavelength dependence of the coupling ratio presents a problem when the branching component is used in a wide wavelength region.
FIG. 2 shows an example of the wavelength dependence of the coupling ratio of the directional coupler type guided-wave branching component in FIG. 1. In FIG. 2, when the coupling ratio is set to 50% at 1.3 .mu.m wavelength, the coupling ratio at 1.55 .mu.m approximates to 100%. This shows that it is impossible for the branching component to operate simultaneously at wavelengths of 1.3 .mu.m and 1.55 .mu.m.
Generally speaking, the power coupling ratio C of a directional coupler is given by the following equation: EQU C=sin.sup.2 .PSI. (1)
where .PSI. depends on a space between the waveguides at the coupling region of the directional coupler, the length of the coupling region, wavelength, etc. In the example in FIG. 2, .PSI. is approximately zero at 1.0 .mu.m wavelength, .pi./4 at 1.3 .mu.m, and .pi./2 at 1.6 .mu.m. As a result, C varies approximately sinusoidally in accordance with the wavelength. This is the reason why the coupling ratio of 50% cannot be maintained in a wide wavelength region in FIG. 2.
Another configuration of a guided-wave branching component is known as a "Y-branching" type. Although the wavelength dependence of the coupling ratio (i.e., branching ratio) of the Y-branching type is small, it has a basic disadvantage in that an optical power loss of more than about 1 dB cannot be avoided at the Y-branching region. In addition, the Y-branching type cannot perform all of the functions or uses of the directional coupler type because the Y-branching type has only three ports whereas the directional coupler type has four ports.
The above describes the problems with regard to a conventional (2.times.2) type guided-wave branching component. Next, problems concerning a conventional (3.times.3) type guided-wave branching component will be described.
FIG. 3 is a planar view exemplifying the configuration of a conventional (3.times.3) type guided-wave branching component. In FIG. 3, three optical waveguides 10, 11, and 12 are formed on a flat substrate 9. A part of each waveguide is brought into proximity with the others so as to form a directional coupler 13. The directional coupler 13 is designed in such a way that an optical signal launched into a port 15 is equally branched to ports 17, 18 and 19 to be outputted. Although the power coupling ratio of the directional coupler 13 can be specified to a desired value at a particular desired wavelength, the wavelength dependence of the coupling ratio presents a problem when the branching component is used in a wide wavelength region.
FIG. 4 shows an example of the wavelength dependence of the coupling ratio of the guided-wave branching component shown in FIG. 3. In FIG. 4, when the coupling ratios are set in such a manner that the optical signal is equally divided to each output port 17, 18 and 19 at wavelength of 1.3 .mu.m (i.e., coupling ratios I.sub.15-17 =I.sub.15-19 =0.33, I.sub.15-18 =0.34), the coupling ratios at 1.55 .mu.m become I.sub.15-17 =I.sub.15-19 =0.45, I.sub.15-18 =0.10. Therefore, the branching component cannot be used as an equal branching component which operates simultaneously at the wavelengths of 1.3 .mu.m and 1.55 .mu.m.
Generally speaking, the power coupling ratio C (=I.sub.15-17 - I.sub.15-19) of a (3.times.3) directional coupler when the optical signal is launched into the center optical waveguide (waveguide 11 in FIG. 3) is given by the following equation: EQU C=(sin.sup.2 .PSI.)/2 (1')
where .PSI. depends on the space between the optical waveguides at the coupling region of the directional coupler, the length of the coupling region, the wavelength, etc. Usually, .PSI. increases with an increase in wavelength. This is the reason why the coupling ratio of 33% (C=33%) cannot be maintained in a wide wavelength region in FIG. 4.
Although the problems with regard to a conventional (3.times.3) optical branching component is described above using the example of a guided-wave type, the fiber type branching components have similar problems.
Next, a conventional optical switch will be described. Optical switches are going to play an important role in the near future, because they are necessary to freely switch optical fiber communication lines to meet demand, or to establish an alternate route during a communication line failure.
The configurations of optical switches are divided into two classes: (1) bulk type; and (2) guided-wave type. These types have respective problems. The bulk type is arranged by using movable prism, lenses, or the like as constituents. The advantage of the bulk type is that the wavelength dependence is small, and the optical power loss is low. However, the bulk type is not suitable for mass production because the assembly and adjusting processes are tedious, and in addition, it is expensive. These disadvantages hinder the bulk-type from being widely used.
In contrast, the guided-wave type optical switches can be mass-produced because integrated optical switches of this type can be constructed with waveguides on substrates by using the lithography or micro-fabrication technique. The guided-wave type is a highly promising type of optical switch.
FIG. 5 is a planar view exemplifying the configuration of a conventional guided-wave type optical switch. In FIG. 5, each of the two 3-dB optical couplers 21 and 22 are formed on substrate 20 as directional couplers, each of which is formed by two optical waveguides 23 and 24 placed side by side in close proximity. The coupling ratio of each 3-dB optical coupler 21 and 23 is specified as 50% (i.e., a half of the complete coupling length) at the wavelength of the optical signal. The optical-path lengths of the two waveguides 23 and 24 which connect the two 3-dB couplers 21 and 22 are set to have the same value when phase shifters 25 and 26 formed midway between the 3-dB couplers are not in operation.
In this condition, an optical signal launched into a port 27 is emitted from an output port 30 and not from an output port 29. In contrast, the optical signal is switched to the output port 29 when at least one of the phase shifters 25 and 26 is operated so as to produce the optical-path length difference of about 1/2 wavelength (that is, an optical phase of 180 degrees or .pi. radian) between the optical waveguides 23 and 24. Thus, the device works as an optical switch. This guided-wave type optical switch is also called a Mach-Zehnder interferometer type optical switch, and can accomplish a switching function by using rather simple phase shifters. For this reason, various waveguides made of different materials including glass have been employed to construct the Mach-Zehnder interferometer-type optical switches. These conventional guided-wave optical switches present the following problems:
FIG. 6 shows a set of characteristic curves representing the wavelength dependence of the coupling ratios between the input port 27 and output port 30 of the optical switch which is designed and constructed to be used at the wavelength of 1.3 .mu.m. Curve (a) shows the coupling characteristics when the phase shifters 25 and 26 are in the OFF state, curve (b) shows the coupling characteristics when one of the two phase shifters 25 and 26 is in the ON state, and curve (c) shows, as a reference, the wavelength dependence of the coupling ratio of respective 3-dB optical couplers which constitute the optical switch.
When one of the phase shifters 25 or 26 is in the ON state (curve (b)) , coupling ratio I.sub.27-30 is approximately zero (below 5%) in a considerable wide wavelength region of about 1.3 .mu.m.+-.0.2 .mu.m. Hence, the optical signal is transmitted through the path (27.fwdarw.29) with little wavelength dependence in this region.
In contrast, when the phase shifters 25 and 26 are in the OFF state (curve (a)), the coupling ratio I.sub.27-30 above 90% is restricted to a narrow region of 1.3 .mu.m.+-.0.1 .mu.m. Outside of this region, for example, at the wavelength of 1.55 .mu.m, the coupling ratio reaches only about 50%. This means that the switching cannot be accomplished appropriately, and this presents a great problem.
The large wavelength dependence of the conventional guided-wave optical switch shown in FIG. 5 mainly results from the following: the 3-dB optical couplers (the directional couplers) exhibit a large wavelength dependence as shown by the curve (c) in FIG. 6; when the coupling ratio is specified to 50% at the wavelength of 1.3 .mu.m as shown by the curve (c), it increases to far above 50% with an increase of wavelength, and hence, the 3-dB couplers cannot accomplish their role.
Optical signals having wavelengths of 1.3 .mu.m and 1.55 .mu.m are often transmitted simultaneously in the optical switches used for switching optical fiber communication lines. Hence, the optical switches having a large wavelength dependence present a great problem in a practical use.
So far, the problem results from the large wavelength dependence of the coupling ratio is has been described with regard to conventional optical branching components and optical switches using the example of optical signals whose wavelengths are 1.3 .mu.m and 1.55 .mu.m which are widely used. In reality, however, the wavelength dependence at the wavelength of 1.65 .mu.m is also a great problem, because the wavelength of 1.65 .mu.m is used as a monitor beam in OTDR (Optical Time Domain Reflectometer) to determine the state of a transmission line on the basis of the back-scattered waveform of the monitor beam sent. Thus, not only are optical signals having wavelengths of 1.3 .mu.m and 1.55 .mu.m simultaneously transmitted through the optical switches, but also the monitor signal having a wavelength of 1.65 .mu.m may be transmitted through the optical switches.